1. Field of the Invention
The present invention relates to engine analyzer apparatus used for testing internal combustion engines. In particular, the present invention relates to apparatus for measuring the internal combustion engine by selectively disabling cylinders while the engine is running.
2. Description of the Prior Art
An internal combustion engine used to power automobiles, trucks, and other land vehicles typically has several cylinders, and has an ignition system which includes a battery, an ignition coil, a condensor, a circuit interrupter (either breaker points or a solid state switching device), a distributor, and spark plugs for each of the cylinders. As the engine runs, the circuit interrupter periodically interrupts current flow through the primary winding of the ignition coil, thus inducing a high voltage output pulse which is supplied by the distributor to one of the spark plugs.
This type of ignition system requires periodic testing and maintenance in order to obtain the desired performance from the engine. One type of test which has been used is sometimes referred to as "Power Check", and involves the shorting out of selected spark plugs and observing the change in rpm of the engine.
The power check test requires a circuit which keeps track of cylinder number and shorts out the circuit interrupter before it opens to interrupt current flow and produce an output pulse to the selected spark plug. U.S. Pat. Nos. 3,572,103 and 3,573,608 describe power check circuitry which utilizes a ring counter and a pulse generating circuit for firing an SCR which shorts out the breaker points.
One difficult technical problem encountered with power check test circuitry is the proper timing of the power check pulse so that it begins before the circuit interrupter opens (i.e. the cylinder fires) and ends after the circuit interrupter closes. This is complicated by instantaneous speed fluctuations of the engine which result in different times between firing of different cylinders.
There is a continuing need for improved power check test circuitry which accurately generates power check pulses having the desired time relationship to the firing of a selected cylinder. In addition, there is a need for improved power check test circuitry which is capable of selectively shorting out individual spark plugs or groups of spark plugs in predetermined patterns.